1. Field of the Invention
Cerebral dysfunction in man is largely directly attributable to increased intracranial pressure. Whether this increase in pressure is precipitated by external injury (such as in the case of accident victims suffering significant head trauma) or internal injury (as in the case of victims of brain tumor, stroke, brain infection, or hydrocephalus) irreversible damage to the brain is caused by increasing intracranial pressure. Because there are both surgical and pharmacologic means of reducing intracranial pressure, a system which can accurately and continuously measure this important parameter with minimal risk and inconvenience to the patient is of significant clinical value.
The invention concerns itself with a minimally invasive biomedical system for continuously, chronically, and accurately monitoring intracranial pressure.
2. Description of the Prior Art
There is recognition in neurosurgical practice of the need to quantitatively measure and control intracranial pressure in ill patients. Several different types of transducers have been introduced for the measurement of intracranial pressure (ICP). Originally, "intracranial pressure" was defined as intraventricular fluid pressure. Over the last decade the definition has been broadened to encompass pressures measured at other locations within the cranium.
The dura mater, commonly called the dura, is a tough, fibrous membrane covering the brain and the spinal cord. Measurement of pressure within the brain has been accomplished by the insertion of cannulae into the lateral ventricles, but this is a difficult and dangerous task and not satisfactory for postoperative monitoring on the nursing ward. The position of the cannulae must be maintained with some precision, and because the dura has been breached, the risk of cerebral infection is markedly increased. The article "Radio Telemetry for the Measurement of Intracranial Pressure," James R. Atkinson et al., Journal of Neurosurgery, Vol. XXVII, No. 5, 1967, pages 428-432, discloses an ICP measuring system utilizing an implanted ventricular catheter with a resonant circuit. As the ICP varies, the resonant frequency of the resonant circuit changes. The frequency of the implanted resonant circuit is measured with a grid-dip circuit. The Atkinson device, in addition to offering a high risk of infection, utilizes a plastic container which has unwanted leakage, and an inductor (in the resonant circuit) which is susceptible to incontrollable variable extraneous capacity effects.
Early in this decade Majors, Schettini, Mahig and Nevis discovered that intracranial pressures could be measured with an epidurally implanted transducer (Medical & Biological Engineering, Vol. 10, 1972, pages 724-733). The accuracy of the measurement depends on the dura being in coplanar approximation with the sensing surface of the transducer. The papers of Yoneda et al. and Rudenberg et al. (Surgical Neurology, Vol. 1, January 1973, page 13; and 24th ACEMB, 1971, page 187, respectively) disclose the use of epidurally-placed pressure transducers. From the standpoint of patient acceptability they are completely unsatisfactory as they require wires to protrude from the scalp.
At the American Association of Neurological Surgeons, Americana Hotel, Bal Harbor, Fla., Apr. 6-10, 1975, messrs. Walker, Viernstein, Chubbuck and Karas described an "Intracranial Pressure Monitor" which comprises an L-C circuit in a cylindrical plastic container. The electrodes of the capacitor are each supported by a movable bellows. One side of the LEXAN.RTM. case, the side placed next to the patient's dura, has a thin LEXAN.RTM. diaphragm. The inductor and capacitor are engulfed in silicon oil. As the diaphragm is depressed, the bellows are compressed and the capacitance of the capacitor is increased. This results in a lowering of the resonant frequency of the parallel resonant circuit formed by the capacitor and inductor. The resonant frequency of the L-C circuit is monitored by an external detector that carries a frequency-modulated VHF signal that loses energy to the L-C circuit when the signal frequency passes through the resonant frequency of the L-C circuit.
The Journal of Neurosurgery, Vol. 44, April 1976, pages 465-478, contains an article by H. Grady Rylander et al. which describes an ICP monitoring system wherein an epidural pressure transducer with a variable inductance is part of the resonant circuit of a tunnel diode oscillator. The transducer comprises a coil with a movable ferrite core therein. The ferrite core is supported by a metal bellows one end of which is secured to the input diaphragm of the transducer. Power for the tunnel diode oscillator is inductively coupled from a power oscillator positioned close to the implanted transducer. A fixed capacitor is connected in parallel with the variable inductor to form a resonant circuit for the tunnel diode oscillator. An antenna coil is coupled in series with the L-C circuit to radiate an r-f signal to a remote receiver. The antenna coil is wound coaxially over the power coil and both coils are contained within a cylindrical Teflon.RTM. housing. When the patient's dura moves the transducer diaphragm, the bellows flexes and varies the inductance of the coil. As the inductance changes, the frequency of oscillation of the tunnel diode oscillator changes. Tests conducted by the authors revealed a considerable discrepancy between epidural pressure and intraventricular fluid pressure a few days after the transducer was implanted.
The Walker and Rylander devices both suffer from similar shortcomings. In each device the resonant circuit is susceptible to extraneous distributed capacity effects which alter the resonant frequency and produces zero drift. LEXAN.RTM. is known to creep and flow and it is not impervious to liquids. In the Walker device, creep in the LEXAN.RTM. diaphragm causes zero drift. Fluids can penetrate the plastic housing of the Rylander monitor and alter the characteristics of the antenna coil. Inasmuch as the antenna coil is not buffered from the oscillator resonant circuit, extraneous distributed capacitance changes and mutual inductance effects in the antenna coil can produce unknown and undesirable deviations in the oscillation frequency of the tunnel diode oscillator. Antenna coil related pressure errors as large as 2 torr are acknowledged by Rylander. In order to achieve adequate inductance changes, the Rylander system must rely on large diaphragm motions. A soft bellows is accordingly dictated. When a soft bellows is utilized, sealing and hysteresis problems can be expected. Rylander indicates that his device has a hysteresis error as large as 0.7 torr. The Rylander and Walker transducers have very compliant diaphragms. The compliant transducers permit the dura to be distended and this results in inaccurate pressure measurements.
Therefore, there has been a recognized but unfulfilled need for an epidural intracranial pressure measuring system that may be used continuously and chronically and is free from hardwires and short-lived batteries and which is further characterized by minimal or no fluid leakage, hysteresis, and zero drift.